1. Field of the Invention
The present invention relates to a secondary battery, and more particularly, to a lithium secondary battery having an improved structure to attain an excellent safety, and a manufacturing method thereof.
2. Description of the Related Art
Lithium batteries are classified into liquid electrolyte batteries and polymer electrolyte batteries according to the type of electrolyte used. Generally, batteries using liquid electrolyte are called lithium ion batteries and batteries using polymer electrolyte are called a lithium polymer batteries.
Lithium secondary batteries are fabricated in various shapes. Typically, cylindrical or prismatic batteries are fabricated as lithium-ion batteries. Lithium polymer secondary batteries have flexibility so that they are relatively free in view of shape design. Accordingly, lithium polymer secondary batteries having excellent safety and freedom in shape design and light in weight are advantageous in attaining miniaturized and light-weight portable electronic apparatuses, and research into the lithium polymer secondary battery is being carried out in various manners.
FIG. 1 is a partially enlarged cross-sectional view of a battery unit 10 in a conventional lithium secondary battery.
Referring to FIG. 1, the battery unit 10 includes a positive electrode plate 11, a separator 13 and a negative electrode plate 12 laminated in sequence.
The positive electrode plate 11 includes a positive electrode current collector 14 having a plurality of openings 14a. The positive electrode current collector 14 is a thin film of aluminum. A front positive electrode sheet 15 and a rear positive electrode sheet 16 are present on both surfaces of the positive electrode current collector 14. The positive electrode sheets 15 and 16 have positive electrode active materials such as lithium oxide as main components and have a binder, a conductive material and a plasticizer as auxiliary components.
The negative electrode plate 12 includes a negative electrode current collector 17, like the positive electrode plate 11. The negative electrode current collector 17 has a plurality of openings 17a and is a thin film of copper. A front negative electrode sheet 18 and a rear negative electrode sheet 19 are fixed to both surfaces of the negative electrode current collector 17. The negative electrode sheets 18 and 19 have negative electrode active materials such as carbon materials as main components and have a binder, a conductive material and a plasticizer as auxiliary components.
The positive electrode plate 11, the separator 13 and the negative electrode plate 12 are fused by applying heat and pressure through a laminating process. The plasticizer is extracted from the positive and negative electrode plates 11 and 12 and an electrolyte is impregnated into a space produced by extracting the plasticizer.
The lithium secondary battery having the above-described battery unit 10 encounters with several problems.
In other words, a lithium ion battery generally uses lithium-based oxide for a positive electrode, carbon-based oxide for a negative electrode and an organic solvent for an electrolyte. Thus, if the battery is overcharged, the electrolyte is decomposed at the positive electrode and lithium metal is precipitated in the negative electrode, thereby deteriorating the battery characteristics. Also, the battery may generate heat or ignition.
A lithium polymer battery is also locally overheated during charging and discharging. Accordingly, a polymer electrolyte which is thermally weak is locally dissolved or softened. Thus, the distribution of electrical potentials is not uniform, which results in short-circuiting.
Thus, in the lithium polymer battery, it is most important to achieve battery safety. To this end, lithium polymer batteries undergo various types of safety tests. Among them, a piercing test is performed in preparation for the event that severe internal short-circuiting may be caused by an external force. If internal short-circuiting occurs, energy is instantaneously concentrated on the spot where the short-circuiting occurs. Then, the battery experiences thermal runaway due to heat and other side reactions. Various researches into methods of removing the dangers of the battery are being carried out.
To solve the above problems, it is an object of the present invention to provide a lithium secondary battery by which internal short-circuiting is prevented by forming a coating layer having an excellent elongation ratio on at least one surface of positive and negative electrode plates to thus improve a battery safety, and a manufacturing method thereof.
Accordingly, to achieve the above object, there is provided a lithium secondary battery including a positive electrode plate having a positive electrode current collector and a positive electrode sheet having a positive electrode active material fixed to at least one surface of the positive electrode current collector as a main component, a negative electrode plate having a negative electrode current collector and a negative electrode sheet having a negative electrode active material fixed to at least one surface of the negative electrode current collector, and a separator interposed between the positive electrode plate and the negative electrode plate, wherein a coating layer made of a polymer material having a high elongation ratio is formed on at least each one outer surface of the positive and negative electrode plates.
Also, the coating layer is a polymer material selected from the group consisting of polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoro propylene (PVDF-HFP), polyvinyl chloride homo-polymer (PVC homo-polymer), polyvinyl chloride co-polymer (PVC co-polymer), styrene-butadiene rubber (SBR), polyacrylonitrile (PAN) and poly(methyl methacrylate) (PMMA).
Further, the coating layer preferably consists of 3 to 30% by weight of a coating solution. Also, the coating layer preferably has a thickness of 1 to 10 micrometers and an elongation ratio of 200% or higher.
According to another aspect of the present invention, there is provided a method for manufacturing a lithium secondary battery including the steps of providing a positive electrode current collector and a negative electrode current collector, fixing a positive electrode sheet and a negative electrode sheet to at least each one surface of the positive and negative electrode current collectors and primarily rolling the same to form a positive electrode plate and a negative electrode plate, forming a coating layer made of polymer having a high elongation ratio on at least each one surface of the positive and negative electrode plates, immersing the positive and negative electrode plates having the coating layer in a separately provided solvent and producing a space where an electrolyte is impregnated, drying the positive and negative electrode plates and secondarily rolling the same to have a predetermined thickness, and interposing a separator between the positive electrode plate and the negative electrode plate and thirdly rolling the same.
In the step of forming a coating layer, preferably, a polymer material selected from the group consisting of polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoro propylene (PVDF-HFP), polyvinyl chloride homo-polymer (PVC homo-polymer), polyvinyl chloride co-polymer (PVC co-polymer), styrene-butadiene rubber (SBR), polyacrylonitrile (PAN) and poly(methyl methaceylate) (PMMA), is coated.
Also, in the step of forming a coating layer, the polymer contained in the coating layer is preferably 3 to 30% by weight. Further, in the step of forming a coating layer, a solvent selected from the group consisting of acetone and N-methyl pyrrolidone (NMP) may be used for dissolving the polymer.